I'm sure somewhere there are kits to build rabbit C&C cages this site (guineapigcages.com) does not offer them as they are focused on guinea pigs. Someone on this rabbit forum may know where to buy kits: https://www.binkybunny.com/FORUM/tabid/54/Default.aspx
This is one of my favoirte ways to build your own rabbit C&C. Keep in mind that this cage was planned out for dwarf or small rabbits.
Oxbow IS a good choice, but you may need to transition your bunny slowly to the new pellet. It is best to mix the old with the new over the period of a week or so.
Also, be sure you have plenty of hay!
(Oxbow has one type of pellet for juveniles and one type for adult rabbits)